1. Field of the Invention
The present invention relates to an equalizing apparatus, and more particularly, to a non-linear equalizing apparatus used for a digital magnetic recording medium.
2. Description of the Related Art
Nonlinearity becomes a serious problem as recording density increases in a digital magnetic recording medium such as a hard disk drive (HDD). The nonlinearity is generated due to interaction between adjacent transitions. A demagnetization field of a previously recorded transition shifts the position of a subsequently recorded transition and increases the width between transitions. The adjacent transitions operate to erase each other. As a result, the size of a reproduced signal is reduced. Such phenomenons are known as nonlinear transition shift (NLTS), a transition broadening, and a partial erasure respectively.
When data are to be recorded, some peripheral bits generate such a nonlinear distortion. However, an interference effect between signals in reproduced signals exists in many bits. Accordingly, the nonlinear distortion affects many bits. When a signal detector does not process the nonlinear distortion, data detection reliability deteriorates.
According to a conventional technology (U.S. Pat. No. 5,132,988), the nonlinear distortion is processed using a feedback RAM instead of a feedback filter. Since a RAM model for processing the nonlinear distortion models only the nonlinear distortion from past data, it is impossible to consider the interaction between current and future data.
Also, since the size of a RAM is doubled as the range of data filtered by a feedback portion increases by one bit in this equalizer, the amount of data which can be processed by the feedback portion is restricted by the size of the RAM.
The linearity and nonlinearity characteristics of a channel are shown by three pulses in a paper by W. Zeng and J. Moon ["A Practical Nonlinear Model for Magnetic Recording Channels", IEEE Trans. Magn., vol. 30, no. 6, pp. 4233-4235, November 1994]. The three pulses are an isolated transition response which is a transition response in the case that there is no transition between adjacent data and dibit responses which are transition responses in the case that there is a transition in a one bit past and there is a transition in a two bit past. The three pulses are obtained by measuring the reproduced signals.
However, a channel characteristic is not obtained from all the data sequences which may be generated but obtained by measuring a channel signal in an extremely restricted situation, in this model. As a result, this model does not provide a method of mathematically optimizing a model in order to heighten the reliability or obtaining the channel characteristic in a more general situation. Also, logic regulations for generating logic variables such as Q.sub.k, R.sub.k, ERA, and CHO required for operating the model are not mathematically induced but are created depending on the experience of the writers. The nonlinear equalizer in which the model provided by W. Zeng and J. Moon is used ["Decision feedback equalizer with pattern dependent dynamic threshold," IEEE Trans. Magn., vol. 32, no. 4, pp. 3266-3273, July 1996] includes all the restrictions and problems which the model has.